1. Field of the Invention
The present invention relates to a semiconductor image sensor, and more particularly to a semiconductor image sensor which is provided with a photosensing and accumulation region of a hook structure.
2. Description of the Prior Art
Semiconductor image sensors that have heretofore been employed are mostly the MOS type and the CCD type. The MOS type image sensor permits simple peripheral circuit arrangements and allows ease in incorporating peripheral circuits and a scanning circuit into the sensor body but, on the other hand, it has the disadvantages of large switching noise and an excessive video line capacitance which leads to lowered sensitivity when it fabricated for multibit operations. In contrast thereto, the CCD type image sensor has high sensitivity and hence can be used at low light levels and, in terms of its arrangement, permits fabrication for multibit operations; in practice, however, when it is constructed for multibit operations, its drive circuit becomes complicated and complexity is introduced in its manufacturing process and, in addition, a high degree of stability is needed in operation.
Recently a novel image sensor which is free from such defects has been proposed by the present inventors. Which image sensor will hereinafter be referred to as the prior art image sensor. The prior art image sensor achieves (1) wide dynamic range, (2) high sensitivity, (3) low noise and (4) high image clarity by the provision of a photosensing and accumulation region of a hook structure. One of its striking features is non-destructive readout of optical information by a carrier storage effect which is characteristic of the hook structure.
In the photosensing and accumulation portion of an example of the abovesaid image sensor, there is formed a hook structure which comprises a low resistivity first region of a first conductivity type, a high resistivity second region, a low resistivity third region of a second conductivity type reverse from the first one and a low resistivity fourth region of the first conductivity which are sequentially formed one on another in a semiconductor substrate inwardly thereof from its surface; namely, a hook structure is constituted by, for example, an n.sup.+ region, a p.sup.- region, a p.sup.+ region and an n.sup.+ region. Of these regions, only the p.sup.+ and n.sup.+ regions on the side of the substrate are isolated by an insulating isolation region in a lateral direction to form a plurality of cells, thereby defining a pn junction. That is, the n.sup.+ and p.sup.- regions on the side of the surface of the substrate assembly are common to the cells. A transparent electrode is formed on the outer n.sup.+ region and, on the other hand, a readout transistor is connected to the inner n.sup.+ region. The substrate is irradiated by light through the transparent electrode which is supplied with a positive voltage.
Electrons of electron-hole pairs which are created by the optical irradiation in the vicinity of the substrate surface, are immediately absorbed by the transparent electrode supplied with the positive voltage but holes are accelerated by the electric field to flow across the p.sup.- region into the inner p.sup.+ region. Between the p.sup.+ region and the inner n.sup.+ region is formed a pn junction having a predetermined barrier voltage. The barrier prevents the holes having flowed into the p.sup.+ region further flow into the inner n.sup.+ region. In consequence, the holes are accumulated in a potential well of the hook structure formed in this p.sup.+ region. As the accumulation of the holes proceeds, the barrier voltage of the pn junction drops, resulting in flowing out of electrons from the inner n.sup.+ region towards the p.sup.- region across the pn junction. The electrons are accelerated to flow through the p.sup.- region and the outer n.sup.+ region and absorbed into the transparent electrode supplied with the positive voltage. As a result of this, the floating inner n.sup.+ region from which the electrons have flowed out is charged positive to raise its potential. The increased potential of this n.sup.+ region is read out by the aforesaid readout transistor.
The prior art image sensor possesses the above-described many advantages over the conventional MOS type and CCD type image sensors but has some problems to be solved for enhancement of light receiving sensitivity, operation speed and so forth.